Stainless steel pipes provide various functions in a multitude of technologies. One function stainless steel pipes currently provide is transporting corrosive fluid, or gases containing chlorine or fluorine etches used in the manufacture of semiconductor devices, from one point to another. Commonly, such corrosive fluids or gases adversely react with microdefects, such as pin holes, boundary junctions and triple points, causing corrosion and eventual rupture of the stainless steel pipe. For safety purposes, it is, therefore, desired to monitor the pin holes and other defects in the structure of the stainless steel pipe during use or as an incoming verification.
In an attempt to standardize the defect detection process and provide a safe work environment, an American Society for Testing and Materials (ASTM) standard for testing stainless steel pipe has been developed. Currently, the ASTM standard method employed to measure the quality of stainless steel pipe is accomplished by taking a microscopic picture at 3500 times magnification and then overlaying a 1 cm square grid on top of the picture. A human then counts the number of boxes in the grid that have defects, assigning a defect density to the stainless steel pipe.
The aforementioned ASTM standard is extensively used, unfortunately, it experiences certain drawbacks. First, the ASTM standard may not focus on a random point of the stainless steel pipe. Focusing the microscope requires that there be a point on which to focus. This requires, in general, that the focusing be done in areas where there is something to focus on, such as an area having a large amount of defects. Since the microscope is typically focused on a point having the highest number of defects, an inaccurate defect count may be obtained.
A second drawback of the ASTM standard stems from inaccuracies resulting from human error. One of such human error inaccuracies is caused by variations in counting style between various people. Where one person counting might record multiple defects in a single square as multiple defects, another person counting might record the multiple defects within the single square as a single defect. A similar situation might occur with a defect spanning multiple squares.
Another inaccuracy resulting from human error, stems from the limited area that may be tested using the human eye. The area that is seen with the microscope is very small with respect to the length and area of the stainless steel tubing. This only provides an average of the defects over an extremely small area, not a complete reading. In essence, the methodology of placing a square grid over a picture is better suited for making a course defect density, rather than for extremely precise measurements, as currently desired in the industry.
Accordingly, what is needed in the art is a method of testing the surface of a stainless steel pipe for defects that does not experience the drawbacks as experienced with the prior art methods.